A common rail fuel supply system includes a reservoir of fuel on the high-pressure typically in the form of an elongate tube having a fuel reservoir which is also known as a main gallery. Fuel injectors, typically one per cylinder, are connected to the common rail at spaced intervals through an injector cup which is typically connected mechanically and hydraulically directly to the common rail. In a known system, the fuel injector is located in the injector cup and is secured in position in the injector cup by means of a connection plate which is coupled to the fuel injector cup through two bolts. This known arrangement worked satisfactorily but has the disadvantage that it requires a significant amount of space in an environment such as in a vehicle where space is extremely limited. There are also difficulties in ensuring that process parameters such as the tightening torque on the bolts is accurately carried out. As a result, this solution is expensive and time consuming to assemble and not suitable for many applications. U.S. Pat. No. 8,479,710 B2 discloses a fuel injector system in which a coupling structure is provided for coupling a fuel injector to an injector cup, in which the cup has diametrically opposite slots in a peripheral wall for receiving clips which engage an injector to locate the injector in the cup. The clips are held in position by a retainer band which is fastened over the clips. The use of these several components is costly, complicates assembly and mitigates against subassembly of the components since the components need to be held in position on the cup for transportation but have to be disassembled for the injector to be inserted. A further problem arises in that it is desirable for the injector to pivot slightly in the cup and to maintain contact with the securing device in order to prevent radial loads on the injector generated by the fuel pressure during the fuel injection phase.